This invention relates to aluminium cells, and particularly though not exclusively to aluminium/air cells. These have an air cathode and an aluminium anode, and the net chemical reaction occurring is EQU 4Al+6H.sub.2 O+3O.sub.2 .fwdarw.4Al(OH).sub.3
The invention also applies to aluminium cell systems where the oxygen is provided by an oxidising agent such as MnO.sub.2 or H.sub.2 O.sub.2 and to aluminium/silver oxide cells (as in U.S. Pat. No. 3,953,239) in which the overall cell reaction is EQU Al+3/2AgO+3/2H.sub.2 O.fwdarw.Al(OH).sub.3 +3/2Ag
The primary cell system, which may be used with either a neutral chloride or alkaline electrolyte may be recharged mechanically by refuelling the cell with further aluminium. This refuelling takes place in minutes, rather than the hours required to recharge a secondary cell.
Cells with alkaline electrolytes have greatly superior performance to those with neutral chloride electrolytes. The alkaline system provides the basis for both high specific energy (e.g. 400 Wh/kg) and power density (e.g. 175 W/kg), where the comparable figures for neutral chloride systems are 220 Wh/kg and 30 W/kg. Both systems have an infinite shelf life provided that the electrolyte is stored separate from the anode.
A commercially successful aluminium cell employing an alkaline electrolyte has been described in EPA 311275. This cell employs pumped circulation of electrolyte past the anode. The circulation is required for two reasons: to remove aluminium hydroxide from the electrolysis region; and to control the temperature of the electrolyte. However, a pump is disadvantageous for obvious reasons; it requires a secondary power source, and greatly complicates the overall cell system.
EPA 265242 describes a similarly pumped system using injected gas to effect circulation of electrolyte.
U.S. Pat. No. 4,507,367 describes an aluminium-air cell in which hydrogen generated by the anode is used to force circulation of electrolyte. This avoids the need for a secondary power supply. But it requires an anode which generates hydrogen and is therefore inherently inefficient and unsafe. An ideal anode which did not generate hydrogen would not be usable in the system. Also, the entire electrolyte, including the aluminium hydroxide precipitate, is caused to circulate.
U.S. Pat. No. 4,925,744 describes an aluminium-air cell in which electrolyte circulation is by convection. But this is a solids-free high current density cell. When the electrolyte becomes saturated with aluminate, the cell output drops to zero and recharging is required.
The idea of using thermal convection to effect electrolyte circulation in aluminium cells has been canvassed. But there are difficulties:
The neutral saline systems are less efficient (85%), corrode in a non uniform, heavily pitted manner, causing premature anode disintegration and the "product" of reaction is a gel (boehmite) which is not easily removed by the convective forces from the electrode chamber. All in all, they are low power, inefficient and low energy density systems as compared with alkaline cells.
So far as alkaline electrolytes are concerned, cells operated at high current density tend to generate more heat than can be dissipated by simple convection. At low current density, conventional aluminium alloys show low coulombic efficiencies; again, large amounts of hydrogen and heat are generated and the heat cannot satisfactorily be dissipated by simply convection.
There is a need for an aluminium cell in which precipitated aluminium hydroxide settles out by gravity without the need for pumped circulation of electrolyte. This invention aims to provide a semistagnant cell which fulfils that need.